(CNN) - Three powerful Democratic congressmen are seeking more information about the recent departures of multiple senior Department of Homeland Security officials, including Secretary Kirstjen Nielsen, as President Donald Trump threatened to shut down parts of the US-Mexico border.

The members of Congress wrote acting Homeland Security Secretary Kevin McAleenan on Thursday requesting all communications related to the departures of Nielsen, her deputy Claire Grady, Secret Service Director Randolph "Tex" Alles, acting Immigration and Customs Enforcement chief Ronald Vitiello and the possibility of others leaving. They requested the documents by May 9.

"We are deeply concerned that the firing and forced resignation of these officials puts the security of the American people at risk," wrote the chairmen.

Reps. Elijah Cummings of Maryland, Jerry Nadler of New York and Bennie Thompson of Mississippi, the heads of the Oversight, Judiciary and Homeland Security committees respectively, wrote that they were concerned White House aide Stephen Miller is taking greater control of the administration's immigration and border security agenda.

Department of Homeland Security press secretary Tyler Houlton said the safety of Americans is not at risk during leadership changes.

"Under no circumstances is the safety or security of the United States at risk during a leadership transition at DHS," Houlton said in a statement. "The Department has successfully overseen the transition between a half-dozen Secretaries over the course of our 16-year history. At this time, the overwhelming majority of the senior positions at DHS are filled with either Senate confirmed or Presidentially Appointed officials or someone pending a current Senate confirmation. Under the Acting Secretary's leadership, every employee at the Department is committed to our mission of safeguarding the American people."

CNN has reached out the White House for response to the letter.

Miller, a hardline conservative, has been one of the more influential voices in the White House and was seen as a driving force behind several of Trump's immigration policies, including the President's idea to send detained migrants to sanctuary cities largely run by Democrats.

This week, the White House informed the House Oversight Committee that Miller would not testify before the panel after it invited him to appear.

In a letter, White House counsel Pat Cipollone wrote that there is a "long-standing precedent" for the White House to decline offers for staff to testify on Capitol Hill. Instead, the White House counsel said Cabinet secretaries and other executive branch officials would make a "reasonable accommodation" for Cummings' questions on immigration policy.